I think it's a Moutain Lion bug, i hope it will be fixed in the next update of Moutain Lion...
But, i have found a workaround for this issue.
With a PDF file with this problem :
- Open the PDF with "Preview"
- Print, button "PDF", "Save to PostScript Format"
- Open the PS file with "Preview" --> Conversion to format PDF
- Save the PDF file
- Reopen this PDF file with Preview
--> Export can be do with Quartz filter (sometime with filter "Gray Tone", Preview crach, but with filter "Reduce File size", no problem
--> If a use "PDF Clerk Pro" to export this PDF with quartz filter, all filters work.
Yep, it crashed Preview and did not succeed in exporting my file as Gray Tone. This is clearly a nasty bug. I hope Apple addresses it soon. I use grayscale PDF files extensively when I send materials to workshop sponsors around the country. If you only need to reduce file size, this seems like a massively inefficient way to do it. I recommend this workaround as a last resort only.